Natural gas reserves include hydrocarbon components such as methane (CH4), ethane (C2H6), propane (C3H8), and butane (C4H10), and inorganics such as carbon dioxide (CO2), hydrogen sulfide (H2S), nitrogen (N2), oxygen (O2), and helium (He). The composition of the gas reserves varies with the source type. Associated gas reserves have lower concentrations of inorganic components, such as CO2, H2S, and N2, than non-associated or shale gas reserves.
Table 1 (taken from multiple sources such as Bullin, K. A. and Krouskop, P. E., “Compositional variety complicates processing plans for US shale gas”, Oil and Gas Journal, Vol. 107, pp. 50-55 (2009), Moulijn, J. A., et al., Chemical Process Technology, John Wiley and Sons Ltd., Somerset, N.J., USA (2013), Rojey, A., et al., Natural gas production processing transport, Editions Technip, Paris, France (1997), and Hea, J. P., “Hydrocarbon wetness, NGL, and sulphur byproducts of natural gases in western Canada”, Technical Report 2352, Geological Suvery of Canada (April 1991)) shows the range of concentrations for the five major components in natural gas reservoirs. For pricing purposes, it is useful to determine the composition-dependent heat of combustion of the varied gases.
TABLE 1All values are in volume %, except for C2H6 concentrationin non-associated gas, which is given in mole %Non-ConstituentsAssociated gasassociated gasShale gasCH444-8327-97 27-97 C2H6 8-210-150-16CO20-90-540-10N20-30-260-65H2S0-30-36—
Downhole gas composition determines the calorific value of the gas reserves; but data on the spatial variability of gas composition is also important for developing zonal production strategies. Additionally, information on the presence of corrosive gases such as H2S and CO2 are desirable for the design and specification of completion and production hardware. Traditionally, the gas composition is determined by sampling down-hole followed by surface laboratory analysis such as gas chromatography. Surface analysis is relatively expensive and slow, and sample integrity has to be ensured in storage. The number of samples that can be transported to the surface is also limited.
Several technologies have been proposed to carry out the downhole compositional analysis of gases. These include technologies based on (near) infrared spectroscopy (see, U.S. Pat. No. 4,994,671 to Safinya et al., U.S. Pat. No. 5,167,149 to Mullins et al., U.S. Pat. No. 5,201,220 to Mullins et al., U.S. Pat. No. 5,266,800 to Mullins et al., U.S. Pat. No. 5,331,156 to Hines et al., U.S. Pat. No. 5,859,430 to Mullins et al., all of which are hereby incorporated by reference herein in their entireties), photoacoustic spectroscopy (see, U.S. Pat. No. 7,520,158 to DiFoggio which is hereby incorporated by reference herein in its entirety), mass spectroscopy, gas chromatography (see U.S. Patent Application Publication No. 2012/0053838 to Andrews et al., which is hereby incorporated by reference herein in its entirety), nuclear magnetic resonance (see U.S. Pat. No. 7,126,332 to Blanz et al., which is hereby incorporated by reference herein in its entirety), and electrochemistry (see, U.S. Pat. No. 8,519,713 to Lawrence et al., which is hereby incorporated by reference herein in its entirety). The most successful of these technologies has been infrared absorption spectroscopy as deployed in the downhole fluid analyzer (DFA) tool of Schlumberger.
The DFA uses two spectrometers with twenty and sixteen channels spanning wavelengths of 400-2100 nm and 1600-1800 nm respectively. Using absorption spectra, the DFA tool can determine the approximate relative fractions of methane, ethane, carbon dioxide, and the collective composition of all other hydrocarbons in the gas stream. See, Dong, C., et al., “New downhole-fluid-analysis tool for improved reservoir characterization”, SPE Reservoir Evaluation and Engineering 11(6), pp. 1107-1116 (2008). The limitation of infrared spectroscopy is that the composition of homonuclear diatomic molecules, e.g., nitrogen, oxygen, hydrogen, and inert gases such as helium cannot be determined. These gases are substantially infrared inactive.